1. Field of the Invention
This invention relates generally to control system theory, and more particularly, to a method of controlling a micro-electro-mechanical MEM mirror motion.
2. Description of the Prior Art
An estimator-based controller associated with micro-electro mechanical system (MEMS) mirror movements contains a mathematical model of the mirror. This model predicts what the position (X1) and velocity (X2) should be for a given control effort; and due to a variety of sources, the model will have some error in the predictions. A standard textbook method of correcting for prediction mismatch is to calculate the “residual”, the difference between the predicted and measured position, and then feed that error back into the underlying states using the following equations, where L1 and L2 determine the extent to which the residual error should affect the state variables X1 and X2.X1=X1+(L1*Residual)X2=X2+(L2*Residual)
Predictions associated with MEM mirrors are problematic however, in that the model is basically a spring; so a mismatch in the model results in a steady-state error in the predictions. When the residual correction is made, the result is a steady state error in the predicted states. This steady state error can cause an undesirable transient response. If, for example, there is a relatively high gain on the velocity error, the control loop will end up applying current to cancel the phantom velocity error. The integral term in the PID (proportional, integral, derivative) controller will eventually compensate for the large apparent velocity error; and when transients settle out, the loop will behave as desired.
When using feedback to move the mirror from one angular location to another (hereto referred to as seeks) however, the large required integrator value will change; and although it will eventually reach its final value, it takes many samples to do so. While it is converging however, the control effort is also changing and the position is not held very tightly. The root of this problem is a mismatch in gain between the model and the mirror.
In view of the foregoing, it would be both desirable and advantageous in the MEM mirror art to provide a method of using residual feedback in a MEMS mirror control loop in a manner that substantially eliminates the steady state error in the predicted states that results from the mismatch between the MEM mirror and the model.